This invention relates to a switching-transistor noise filter which eliminates objectionable electrical noise pulses from being transmitted from an open switching wire attached to an intermittently used brush of a multi-speed, brush-type electric motor. In particular, transmission of electrical brush noise from wires which are electrically activated only during certain modes of motor operation, such as a high-speed mode, is minimized over a major portion of the electrical frequency broadcast system.
Brush-type electrical motors with multiple-speed brush connections are commonly used to drive tape recording apparatus. Such motors commonly have at least one intermittently energized brush with a conductive switching wire connected thereto. During operation of said motor in the fast-forward or high speed mode, for example, current is caused to flow through said switching wire and the fast-forward brush connected thereto by activation of a mechanical switch commonly located on the control panel associated with the chassis on which the motor and tape deck are mounted. However, during those periods of time during which the fast-forward mechanical switch is not activated, the switching wire connected to the fast-forward brush of the motor and extending to said open switch acts as a transmitting antenna for voltages present in the moving commutator during normal speed operation. The voltages present in the moving commutator are chopped into pulse form by the intermittent contact with the fast forward brush. The voltage pulses constitute electrical brush noise and contain frequencies throughout the spectrum including the standard allocated frequencies used for AM and FM broadcast and reception.
A problem resulting from the described transmission of electrical brush noise is particularly apparent during operation of a tape recorder for the purpose of recording and replaying signals transmitted by AM or FM commercial broadcasting stations. The electrical brush noise transmitted from the above-described switching wire is received and recorded along with the signal received and recorded from one of said broadcasting stations.
The method used to prevent transmission and therefore undesired reception of such brush noise has heretofore been through use of low-pass, inductance-capacitance filters. Capacitance filtering alone has been found insufficient to prevent the relatively low-frequency AM band frequencies of brush noise from reaching the open lead. Resistance-capacitance filtering is not feasible because of the adverse effect on the voltage-speed characteristic of the motor. In addition, existing LC filter design requires, for proper operation, an energy absorbing output load of given impedance rather than the open-circuited switching wire comprising the load in this case. Furthermore, the magnetic fields surrounding inductors used in such filters tend to cause additional transmission of undesired electrical brush noise, resulting in a counter-productive solution to the problem.